The present invention relates generally to orthopedic surgery and, in particular, to interbody cages and dowels of the type for use in interbody spinal fusions.
With respect to spinal surgery wherein one or more vertebrae are fused, the use of bone dowels have certain advantages over metal cages. First, allograft bone readily fuses to the vertebrae. Second, it is often impossible to determine if metal bone-filled cages have fused to adjacent vertebrae, because the metal obstructs x-ray imaging of the bone within the metal cages as well as the cage vertebra junction. Third, bone dowels have a modulus of elasticity closer to that vertebrae. Consequently, bone dowels stress shield less than metal cages.
Bone dowels have certain disadvantages when compared to metal cages, however. Allograft bone incorporates into host bone through a process known as xe2x80x9ccreeping substitution.xe2x80x9d Host blood vessels grow into the allograft bone in the first stage of this process. Bone removing cells known as osteoclasts then invade the allograft bone. After sufficient bone is removed by the osteoclasts, bone building cells known as osteoblasts lay down new host bone on the allograft bone.
This remodeling process may go on for years. As would be expected, the allograft is weakened by the channels formed by the blood vessels, as well as the bone removal by the osteoclasts. Although the allograft regains its strength once sufficient new bone is formed, allograft bone dowels are at risk of fracture during the period of time that they are weakened. Allograft bone dowel fracture is well known to those skilled in the art of spinal surgery. Bone dowels are also weaker than metal cages, even before they undergo creeping substitution. Consequently, bone dowels can fracture during surgical placement. Fractured dowels can be difficult to remove, and may lead to failure of a fusion to occur. The properties of bone also do not allow certain shapes or machining.
This invention improves upon existing bone dowels through the use of rigid, preferably metallic end plates, thereby providing the advantages of bone dowels while eliminating the disadvantages, as discussed above. The plates are positioned relative to the anterior and posterior end surfaces of the bone dowel, which may be an existing, modified, or a specially fabricated bone section.
The completed structure may be inserted, removed, and positioned into the vertebrae in the same manner as existing devices, that is, by way of a threaded or frictional fit. However, the use of the rigid endplates facilitates load sharing which helps to prevent graft fracture.
In the preferred embodiment, the end plates are positioned against the end surfaces of the bone section through the use of a link member, which passes through the interior of the bone section and connects the end plates. One of the end plates may include a threaded bore, and one end of the link member may be threaded to receive the threaded bore of that end plate to hold it in place. The use of a link member allows a plurality of bone sections to be journaled onto the link member, with a disk of rigid, preferably metallic material to be interposed between each bone section.
Alternatively, the end surfaces of the bone section may each include an aperture having an inside diameter, with each end plate having an outside diameter corresponding to the inside diameter of the apertures, enabling the end plates to be positioned against the end surfaces of the bone section through a frictional fit.
At least the bone section preferably includes an exterior threading, such that with a generally cylindrical outer shape the finished structure may be screwed into place. Alternative geometries, including rectangular, trapezoidal, and so forth may also be accommodated, with and without exterior threading or serrading.